1. Field of the Disclosure
Without limitation, this disclosure relates to compositions and methods for detecting and quantifying the expression of insulin receptor isoform A (IR-A) and/or insulin receptor isoform B (IR-B) in a sample, such as a tissue sample. The disclosure also relates to methods of diagnosis and classification based at least in part upon detecting and quantifying the expression of insulin receptor isoform A (IR-A) and/or insulin receptor isoform B (IR-B) in a sample, such as a tissue sample. Methods of treating a subject based upon such a classification are among additional aspects of the disclosure presented herein.
2. Description of the Related Art
The insulin receptor (INSR) is a transmembrane tyrosine kinase receptor implicated in regulation of energy metabolism. The INSR comprises two subunits, α and β, expressed from a single INSR gene. Two isoforms, designated IR-A and IR-B, are the result of alternative mRNA splicing. IR-A is generated by an alternative splicing of the mRNA transcribed from the INSR gene to omit exon 11, which is included in mRNA of IR-B. (FIGS. 1A-1B). Thus, IR-A differs from IR-B because it lacks a stretch of amino acid residues at the carboxy terminus of the INSR α-subunit. IR-A and IR-B are more than 99% identical (FIG. 2). While the expression profiles of the two isoforms are different, the isoforms are coexpressed in cells. The relative abundance of IR-A and IR-B is regulated by development stage- and tissue-specific factors.
For example, IR-A is predominantly expressed in fetal and cancer cells, whereas IR-B is predominantly expressed in differentiated insulin target cells. IR-A exhibits high affinity for insulin, intermediate affinity for IGF-II, and low affinity for IGF-I. IGF-II binds to IGF type I receptor (IGF-IR) and to IR-A with similar affinities. IR-B is a highly specific receptor for insulin.
Currently methods of determining IR-A and IR-B levels in a patient sample are hampered by the lack of an efficient and accurate means of detection. To date, the only available method to specifically measure IR-A expression has been described by Sciacca, et al (Oncogene 21(54):8240-50; 2002 Nov. 28). This method is based on PCR and gel separation, followed by qualitative measurement of the resulting bands. This method is very labor intensive and is not quantitatively accurate, which limits its use in a high throughput or clinical setting.